Devices such as integrated circuits are complex structures made of a variety of materials. They are fabricated from the sequential formation of alternating and interconnecting bands of conductive, semiconductive and nonconductive layers on an appropriate substrate (e.g., silicon wafer) that are selectively patterned to form circuits and interconnections to produce specific electrical functions.
Photolithography is a commonly practiced process of creating a patterned mask on the surface of a semiconductor wafer so that subsequent patterned processes may be performed. Typically these subsequent patterned processes involve the addition or subtraction of a material by deposition, implant doping, or plasma etching.
Frequently, the pattern is transferred from an exposure mask to the wafer using a photoresist layer and optical lithography exposure tools. A positive or negative image of the desired configuration is first introduced into the resist by exposing it to patterned radiation which induces a chemical change in the exposed portions of the resist. This chemical change is then exploited to develop a pattern in the resist, termed a “latent image.” This pattern is then transferred into the substrate underlying the resist.
A variety of resist materials are employed in lithographic processes for device fabrication. One class of such resist materials includes chemically amplified photoresists. A chemically amplified photoresist is a photoresist to which an acid catalyst reaction is applied. The resist contains a polymer that has certain functional groups, for example, alcohol (OH), phenol (C2H5OH), carboxylic acid (COOH), and the like. A certain portion of these functional groups are “masked”, i.e., the hydrogen atom is removed and replaced by moieties referred to as protecting groups. These protecting groups are removable from the polymer by acidolysis or hydrolysis.
The resist materials also contain an energy-sensitive material in combination with the polymer. When exposed to a certain energy (energy of a particular wavelength) or type (i.e., electron beam), a moiety is generated from the energy-sensitive material which effects the cleavage of the protecting group, thereby “unmasking” the functional group in a “deprotection” reaction. When the protecting group is an acid labile group, i.e., it is removed in the presence of acid, the energy sensitive material is typically a photoacid generator (PAG). The greater the number of protecting groups that are cleaved from the polymer, the greater the chemical contrast between the polymer exposed to radiation and the polymer not exposed to radiation. This chemical contrast between the unexposed resist material and the exposed resist material is exploited to develop a pattern in the resist material.
The chemical contrast is typically a difference between the solubility of the exposed resist compared to that of the unexposed resist in a developer solution, which is typically an aqueous alkali solution. In the case of positive resists, for example, those areas of the photoresist that were not exposed to activating radiation are generally not soluble in alkali, thereby providing a dissolution differential between exposed and unexposed regions during development. Following development, the surface of the semiconductor substrate can be selectively etched by using the photoresist pattern described above as a mask.
During exposure of the resist to energy, as described above, there is a tendency for the acid that effects deprotection to diffuse from the exposed resist into the unexposed resist. Such diffusion, if significant enough, will erode the resolution of the patterned features. The result is features that do not have the desired dimensions being transferred to the wafer. When the diffusion is extreme, the latent image in the resist can be destroyed.
Accordingly, it is desirable to provide a method for enhancing resolution of a chemically amplified photoresist. In addition, it is desirable to provide a method for generating on a substrate a resist image having well-defined dimensions. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.